Refrigeration process

ABSTRACT

A refrigeration process employing a single- or plural-stage flashing of a refrigerant liquid to obtain high and/or intermediate temperature cooling, and condensation of refrigerant vapor and subsequent flashing thereof to obtain low temperature cooling, of a heat exchange zone so as to remove heat from said zone. In one embodiment, a gas is passed through said heat exchange zone in heat exchange relationship with said refrigerants and at least partially liquefied.

United States Patent [19] Harper et al.

[ REFRIGERATION PROCESS [75] Inventors: Ernest A. Harper; Dunn M.Bailey,

both of Bartlesville, Okla.

[73] Assignee: Phillips Petroleum Company,

Bartlesville, Okla.

221 Filed: Sept. 28, 1970 [21] Appl. No.: 75,992

[52] US. Cl 62/9, 62/11, 62/40, 62/28 [51] Int. Cl. F25j 1/00, F25j1/02, F25j 3/02 [58] Field of Search 62/9, 11, 23, 24, 27, 28, a

[-56] v References Cited v 2 UNITED STATES PATENTS 3,274,787 9/1966Grenier 62/23 3,418,819 12/1968 Grunberg 62/40 [4 1 May 7,1974

3,548,606 12/1970 Kuerston 62/40 2,960,837 11/1960 Swenson 62/403,367,122 2/1968 Tutton 62/28 3,485,886 12/1969 Mitchel1.... 62/283,596,473 8/1971 Streich 62/40 Primary Examiner-Norman Yudkoff AssistantExaminer-Arthur F. Purcell [57] ABSTRACT A refrigeration processemploying a singleor pluralstage flashing of a refrigerant liquid toobtain high and/or intermediate temperature cooling, and condensation ofrefrigerant vapor and subsequent flashing thereof to obtain lowtemperature cooling, of a heat exchange zone so as to remove heat fromsaid zone. In one embodiment, a gas is passed through said heat exchangezone in heat exchange relationship with'said refrigerants and at leastpartially liquefied.

20 Claims, 2 Drawing Figures PIPELINE FROM FIG. 2

PATENTEDW "I 197 SHEET 1 or 2 INVENTORS E. A. HARPER D.M. BAILEY ow mmmm W wzjwnmm R 7 A T TORNEYS REFRIGERATION PROCESS This inventionrelates to a refrigeration process.

Various refrigeration processes or cycles are known in the art. ln manyinstances, these refrigeration processes or cycles have been developedfor particular purposes. In all instances, it is of the utmostimportance to obtain maximum efficiency in the refrigeration cycle so asto keep investment and operating costs at a minimum. This isparticularly important, and difficult, when one is cooling and at leastpartially liquefying a gas such as natural gas.

A typical natural gas, while composed predominantly of methane, willcontain significant amounts of higher boiling hydrocarbons. Thiscomplicates the problem of cooling and liquefying the natural gasbecause heat must be removed from the, gas over a wide temperaturerange. Consequently, many of the prior art processes are inefficient andthus uneconomical from the standpoint of investment cost and/oroperating cost. One prior art method uses the residue gaseous methanestream as the refrigerant. Said residue methane stream is passed throughan expansion turbine, thereby lowering its pressure and temperaturesufficiently that it can be employed to cool the incoming feed gasstream in countercurrent heat exchangers. In other prior art methods,external refrigeration systems are employed. Depending upon thetemperature reduction desired, more than one refrigerant is frequentlyemployed in the well known cascade systems. Such a system requiresseparate compressors for each refrigerant.

The present invention provides a more efficient external refrigerationsystem which is capable of cooling natural gas to a very low temperaturewithout the need for separate refrigerants and separate compressors.Thus, the refrigerant employed in the practice of the invention can be asingle-component refrigerant, for example, as essentially purehydrocarbon such as ethane or propane, a freon, etc. However,multicomponent refrigerants are preferred for use in the refrigerationsystem of the invention because they are capable of being employed toeffect cooling of a material over a wider range of temperature than ispossible with single-component refrigerants. This is possible withmulticomponent refrigerants because the composition of suchrefrigerants, as well as the pressure, is changed during therefrigeration cycle in order to obtain the de sired refrigerationtemperature (boiling point of the refrigerant). One presently preferredrefrigerant which can be employed in the practice of the inventioncomprises a mixture of ethane and propane. However, depending upon therefrigeration temperature desired, it is within the scope of theinvention for said refrigerant to contain methane, nitrogen, helium, orother gases more volatile than ethane;

An object of this invention is to provide an improved refrigerationcycle or process. Another object of this invention is to provide animproved refrigeration cycle or process wherein both a refrigerant vaporand a refrigerant liquid are employed for more efficient refrigerationin the cooling of a heat exchange zone for the removal of heattherefrom. Another object of this invention is to provide an improvedmethod for cooling and/or liquefying a natural gas. Another object ofthis invention is to provide an improved method for recovering higherboiling components from a natural gas. Another object of this inventionis to provide an improved method for recovering a refrigerant mixturefrom a natural gasLOther aspects, objects, and advantages of theinvention will be apparent to those skilled in the art in view of thisdisclosure.

Thus, according to the invention, there is provided a method forremoving heat from a heat exchange zone and the contents thereof,utilizing refrigerant vapor and refrigerant liquid streams obtained froma compressed and partially liquefied refrigerant, which methodcomprises: passing a stream of said refrigerant vapor through a firstregion of said heat exchange zone in indirect heat exchange relationshipwith at least one stream of sai refrigerant liquid having a temperatureless than the temperature of said refrigerant vapor so as to cool saidvapor; passing said cooled refrigerant vapor through a succeedingdownstream region of said heat exchange zone in indirect heat exchangerelationship with another stream of refrigerant, obtained as describedhereinafter, and at a temperature less than the temperature of saidrefrigerant liquid, so as to further cool and liquefy said refrigerantvapor; flashing said liqueiied refrigerant vapor so as to further coolsame and to obtain said another refrigerant; and removingfrom said heatexchange zone vapors resulting from the vaporization of saidrefrigerants so as to remove hea from said zone.

Further according to the invention, there is provided a process forrecovering a mixture comprising ethane and propane from natural gas,which process comprises, in combination, the steps of: (a) cooling saidnatural gas under pressure and temperature conditions sufficient topartially liquefy same; (b) fractionating said partially liquefiednatural gas from step (a) in a first fractionation zone to recover anoverhead stream comprising methane, ethane, and propane, and a bottomsstream comprising some propane and higher boiling hydrocarbons; (0)partially condensing said overhead stream from step (b); (d) returning afirst portion 'of said condensed overhead from step (c) to said firstfractionation zone as. reflux thereto; (e) passing-a second portion ofsaid condensed overhead from step (c) toa second fractionation zone asfeed thereto; and (I) in said second fractionation zone, fractionatingsaid feed to recover an overhead stream comprising methane, and abottoms stream comprising ethane and propane as one product of theprocess.

Any suitable refrigerant, either a single-component refrigerant or amulticomponent refrigerant, can be employed in the practice of theinvention. However, multicomponent refrigerants are usually preferred.One presently preferred refrigerant is a multicomponent refrigerantcomprising a mixture of ethane and propane containing from about 5percent to about percent ethane, and from about 95' percent to about 5percent propane. This refrigerant finds wide application in the recoveryof ethane and heavier hydrocarbons from natural gas. Said preferredrefrigerant can contain components higher boiling than propane, e.g.,butane and/or pentanes, usually present in amounts of from about 0.1 to5 per cent. However, the present of said higher boiling components isusually not preferred. Depending upon the degree of cooling desired,e.g., whether or not it is desired to completely liquefy a natural gas,said preferred refrigerant can contain from 10 to 50 per cent of acomponent more volatile than ethane, such as methane, helium, nitrogen,etc.

FIG. 1 is a schematic flow sheet diagrammatically illustrating severalembodiments of the invention.

FIG. 2 is a schematic flow sheet diagrammatically illustrating anotherembodiment of the invention.

Referring now to the drawings, wherein like reference numerals have beenemployed to denote like elements, the invention will be more fullyexplained. It will be understood that many valves, pumps, controlinstruments, and other conventional equipment not neces sary forexplaining the invention have been omitted for the sake of brevity. Forconvenience, and not by way of limitation, the invention will bedescribed with reference to the partial liquefaction of a stream ofnatural gas so as to be able to separate ethane and higher molecularweight components from methane and other components more volatile thanethane contained in said natural gas. The ethane and heavier componentsof natural gas are oftentimes referred to as natural gas liquids. Theseliquids include hydrocarbons such as ethane, propane, butanes, pentanes,and sometimes higher molecular weight components, which are valuable asraw materials for preparing various petrochemicals. The more volatilecomponents referred to above include, in addition to methane, suchmaterials as hydrogen, nitrogen, helium, and the like. The natural gasfeed stream may sometimes be obtained as the effluent from a naturalgasoline plant and will have had at least a portion of the heavierhydrocarbons, water, and carbon dioxide removed. In the calculatedexample used hereinafter in connection with the description of thedrawings, the natural gas stream has been dehydrated to a -l F. dewpoint by conventional means not shown, and contains less than 0.02 molper cent carbon dioxide. While not shown in the drawings, it will beunderstood that if a wet gas stream is to be refrigerated for theliquefaction thereof, and/or the recovery of heavy hydrocarbonstherefrom, provision should be made to dehydrate the gas for waterremoval and to withdraw compounds such as benzene and carbon dioxidewhich would solidify at the low temperatures employed. Such materials inliquid state can be tapped off from the heat exchangers at appropriatepoints of temperature and pressure. It should also be understood thatthe various operating conditions given herein in connection with thedescription of the drawing are not to be construed as limiting on theinvention. Said conditions can be varied depending upon the gas or othermaterial being cooled, the amount of cooling desired, and in the case offractionators the separation to be effected. The drawings will bedescribed with reference to a calculated example using a typical naturalgas having the composition:

Mol Percent Carbon Dioxide 0.40 Nitrogen 0.91 Methane 85.32 Ethane 6.78Propane 3.50 lsobutane 0.57 n-butane 1.33 lsopentane 0.39 n-pentane 0.47Hexanes +Total: 100.00

In one embodiment of the invention, a natural gas stream at atemperature of about 90 F. and a pressure of about 550 psig is passedvia conduit into heat exchanger 12 wherein it is cooled to a temperatureof about 75 F. by indirect heat exchange withthe cold refrigerants andemploying the refrigeration cycle of the invention as describedhereinafter. Said heat exchanger 12 can be any suitable type of heatexchanger. As here illustrated diagrammatically, said heat exchangercomprises a plate-type exchanger wherein the various segments of theexchanger are all housed in one shell. Such exchangers are conventionaland well known in the art. However, it is within the scope of theinvention to employ three or more individual exchangers instead of aunitary exchanger as illustrated. It is also within the scope of theinvention to employ tube and shell-type exchangers instead of aplate-type exchanger.

The reduction in temperature of said natural gas stream during itspassage through heat exchanger 12 causes about 20 mol per cent of thegas to liquefy. About 50 per cent of the ethane, about per cent of thepropane, and all of the heavier hydrocarbons will be liquefied under theabove-described conditions. The

partially liquefied stream is withdrawn from heat exchanger 12 viaconduit 14 and passed into phase separator 16. The noncondensed gascomprising methane is returned to said exchanger 12 via conduit 18 forcountercurrent heat exchanger with the incoming gas stream in conduit10. The now warmed residue gas exits from said heat exchanger viaconduit 20 for delivery to a pipeline or other use. lf desired, saidresidue gas can be withdrawn from the system via conduit 22 instead ofbeing returned to said heat exchangers 12. If desired, the liquefiedhydrocarbons in separator 16 can be withdrawn therefrom via conduit 24.Preferably, said liquefied hydrocarbons are passed through expansionvalve 26 for flash vaporization of a portion thereof by r ediic ing thepressure to about 410 psig and the temperature to about 8 1- F. Theresulting mixture of liquid and vapor is passed into phase separator 28wherein the two phases are separated as indicated in the drawing. Saidtwo phases are withdrawn from separator 28 separately and, preferably,are recombined prior to being returned to heat exchanger 12 for passagethrough at least a portion thereof. The use of a separator such asseparator 28 is preferred in order to distribute the liquid phaseuniformly with the gas phase when two phases are being passed throughthe heat exchanger. Here, and elsewhere, where a liquid and vapor areboth introduced into a heat exchanger it may be desirable to employ aliquid-vapor distributor-such as described. in US. Pat. No. 3,158,010,issued Nov. 24, 1964.

The partially warmed, recombined stream is withdrawn from heat exchanger12 via conduit 30 and passed into deethanizer 32. A stream comprisingessentially all the methane and ethane is removed overhead from saiddeethanizer and, preferably, is returned to said heat exchanger 12 viaconduit 34 for additional heat exchange with the incoming gas stream inconduit 10. Said residue gas in conduit 34 can be removed from exchanger12 via conduit 36, recompressed to pipeline pressure by compressor 38,and combined with the residue gas in conduit 20. If desired, saidresidue gas can be removed from the system through conduit 40 prior tocompressor 38 or through conduit 42 after compressor 38, instead ofpassing same into the pipline. The bottoms from deethanizer 32 comprisespropane and heavier hydrocarbons and are recovered as one product-of thesystem via conduit 44.

In the presently preferred refrigeration cycle employed to refrigeratethe natural gas stream just described, a multicomponent refrigerantcomprising a mixture of ethane and propane is advantageously employed.ln said preferred refrigeration cycle, there is employed two-stageflashing of a refrigerant liquid to obtain high and intermediatetemperature cooling and single-stage condensation of refrigerant vaporto obtain low temperature cooling. Said mixture of ethane and propane iscompressed in the compressor system illustrated to a pressure of about220 psig, cooled to a temperature of about 95 F. in water cooledexchanger 46 and passed via conduit 48 into surge or storage tank 50wherein it separates into a liquid phase and a gaseous phase. The liquidphase contains about 13 molper cent ethane, the remainder being propane.The vapor phase contains about 31 mol per cent ethane, with theremainder being propane. Thus, two refrigerant streams are madeavailable by partial liquefaction of a single, compressed refrigerantstream. The two refrigerant streams have different boiling points (atthe same pressure) by virtue of having different compositions andtherefore are advantageously used at different temperature levels in therefrigeration cycle.

Refrigerant vapor is withdrawn via conduit 52 and introduced into heatexchanger 12 wherein it is cooled to a temperature of about 75 F. andcompletely liquefied by heat exchange with colder refrigerant streams asdescribed hereinafter.

Refrigerant liquid is withdrawn from storage or surge tank 50 viaconduit 54, passed through expansion valve 56 wherein it is expanded orflashed to form a liquid and vapor, and introduced into separator 58 forphase separation. In passing through said expansion valve 56 thepressure on said refrigerant liquid is reduced to about 91 psig and thetemperature is reduced to about 45 F.

Refrigerant liquid is withdrawn from separator 58 via conduit 60 and aportion thereof is passed via conduit 62 into a first region (b) ofsaidheat exchanger 12 wherein it is passed to indirect heat exchangerelationship with said stream of refrigerant vapor introduced viaconduit 52 and said natural gas introduced via conduit 10. The resultingvaporized refrigerant is withdrawn from heat exchanger 12 via conduit64, passed through conduit 65 to knockout drum 66, compressed in thehigh stage 68 of the three-stage compressor system illustrated, thenthrough cooler 46 wherein it is partially liquefied, and then passed viaconduit 48 into surge or storage tank 50, previously described.Refrigerant vapor in separator 58 is withdrawn therefrom via conduit 70and introduced into conduit 65 for passage through said high stagecompression zone 68, previously described, to complete the cycle forboth the liquid and vapor from separator 58. It is desirable to maintaina substantially constant ratio between the refrigerant vapors flowing inconduits 64 and 52 because the refrigerant in conduit 64 (vaporizedrefrigerant from conduit 62) contributes materially to the cooling andcondensation of the refrigerant vapor in conduit 52. Thus, should theflow in conduit 52 increase, the flow in conduit 64 should beproportionally increased. The desired ratio control of said two streamscan be conveniently accomplished by employing ratio controller 74 toadjust or regulate valve 72 in conduit 70 responsive to the flow in saidconduits 52 and 64. For example, if the flow in conduit 64 drops below aspecified ratio of that in conduit 52, said valve 72 will be closedsomewhat, thus increasing the pressure in conduit 62 with resultingincreasein flow in conduit 64 until the desired ratio is reestablished.

A second portion or stream of said refrigerant liquid from separator 58is passed from conduit 60 through expansion valve 76 wherein it isexpanded or flashed to reduce its pressure to about 24 psig and itstemperature to about 7 F., and then passed into phase separator 78wherein a phase separation of the partially vaporized refrigerant iseffected. Liquid refrigerant in separator 78 is withdrawn therefrom viaconduit 80 and introduced into a second region (c) of said heatexchanger 12, downstream from said first region (b) with respect to theflow of said refrigerant vapor through conduit 52. Said refrigerantliquid from conduit 80 is passed in indirect heat exchange relationshipin exchanger 12 with said stream of refrigerant vapor in conduit 52 andsaid stream of gas introduced via conduit 10 so as to vaporize saidliquid refrigerant and further cool said stream of refrigerant vapor andsaid stream of gas. Vapors resulting from the vaporization of therefrigerant in conduit 80 are withdrawn from said heat exchanger 12 viaconduit 82 and passed via conduit 84 and knockout drum 86 into theintermediate stage 88 of the compressor system shown. Effluent fromcompression stage 88 is passed through cooler 80 for partial cooling andthen combined with the refrigerant vapors in conduit 65 for furthercompression therewith in compression stage 68 and return to said storagezone or surge tank 50 as previously described. Refrigerant vapor inseparator 78 is withdrawn therefrom via conduit 92 and combined inconduit 84 with the vapors from conduit 82 for compression, aspreviously described, to complete the cycle for both the liquid andvapor from separator 58.

Returning now to the stream of refrigerant vapor introduced into heatexchanger 12 via conduit 52, said stream is completely liquefied duringits passage through heat exchanger 12, is withdrawn from said heatexchanger via conduit, 94, passed through expansion valve 96 wherein itspressure is reduced to about 4 psig and its temperature is reduced toabout 80 R, and then introduced into phase separator 98 wherein a phaseseparation between the two phases is effected. The liquid and vaporphases in separator 98 are preferably withdrawn therefrom separately andrecombined as shown, and then introduced into said heat exchanger 12 viaconduit 100 for passage through a third region (d) of said heatexchanger, downstream from said second region (0) with respect to theflow of said refrigerant vapor in conduit 52 and the natural gas inconduit 10, and passing said refrigerant in indirect heat exchangerelationship with said streams in conduits 52 and 10 so as to vaporizesaid refrigerant in conduit 100, further cool and liquefy therefrigerant vapor in conduit 52, and further cool and partially liquefythe natural gas in said conduit 10. Refrigerant vapor is withdrawn fromsegment or region (b) of said heat exchanger 12 via conduit 102, passedthrough knockout drum 104, and compressed in low stage 106 of thecompressor system shown. The effluent from low stage compression 106 iscombined with the feed to intermediate stage compression 88 for furthercompression, cooling, and return to said storage tank or surge tank 50,as previously described to complete the cycle.

It is important that the refrigerant mixture in separator 98 be at thelowest temperature in the system, for example about 80 F., in order tocool the gas stream in conduit 10 to about 75 F. In order to control therefrigerant temperature in said separator 98 at said desired 80 F., saidexpansion valve 96 is adjusted or regulated by flow recorder controller108 responsive to the gas flow in conduit 52 as measured by flow meter110 and transmitted via. transmitter l 12. The set point of said flowrecorder controller 108 is in turn reset or regulated by temperaturerecorder controller 114 responsive to the measured temperature of thestreamin conduit 14 as transmitted by transmitter 116. Thus, the desiredtemperature of the refrigerant in separator 16 is obtained byapplyingthat temperature as the set point to temperature recorder controller114.

The above-described refrigeration cycle employing two-stage flashing ofa refrigerant liquid to obtain high and intermediate temperaturecooling, and condensation of refrigerant vapor with subsequent flashingthereof to obtain low temperature cooling, represents the second stageflashing through expansion valve 76 is omitted. In such instances, heatexchange zone 12 would comprise only two regions, e.g., regions (b) and(d) (extended if desired or necessary to cover the entire zone), andregion (0) would be omitted. Appropriate changes in the compressionportion of the cycle can be' made in view of the'disclosure herein.

In other instances where it is desired to cool a gas stream or othermaterial over a more extended temperature range, the liquid refrigerantfrom storage or surge tank 50'can be serially flashed three or moretimes to provide refrigeration at three or more temperature levels priorto the lowest temperature level, e.g., region (d). In such instances,heat exchange zone 12 would comprise more than three regions, e'.g.,regions (b),

(c-l (0-2), and (d), etc. Again, appropriate changes can be made in thecompression portion of the cycle.

In still other embodiments of the invention, the refrigerant vaporsresulting from the liquid refrigerant introduced into segment or region(c) of heat exchanger 12 via conduit 80, instead of being withdrawn fromsaid heat exchanger via conduit 82, can be withdrawn from said heatexchanger via conduit 118 and introduced into conduit 84 for compressionand further handling to complete the cycle as previously described.Similarly, if desired, vapors resulting from the refrigerant introducedinto said heat exchanger 12 via conduit 100, instead of being withdrawnfrom said heat exchanger via conduit 102, can be withdrawn from saidheat exchanger via conduit 120and then introduced into conduit 102 forcompression and further handling to complete the cycle as previouslydescribed.

In still another embodiment of the invention, the refrigerant liquidwithdrawn from storage or surge tank 50 via conduit 54 is divided intotwo portions. A first portion is passed through expansion valve 56 intoseparator 58. The refrigerant liquid in separator 58 is withdrawntherefrom via conduit and passed via conduit 62 into segment or region(b) of heat exchanger 12, as previously described. The second portion ofthe refrigerant liquid in conduit 54 is passed via conduit 55, expansion valve 57 wherein it isexpanded or flashed and its temperaturereduced to about 50 F. and its pressure reduced to about 95 psig, andthen introduced into phase separator 59. Refrigerant vapor fromseparator 59 is withdrawn therefrom and passed via conduit 61 intoconduit 70 for further handling as previously described. Refrigerantliquid from said separator 5 is passed via conduit 63, expansion valve76 wherein its pressure is lowered to about 24 psig and its temperatureis lowered to about -7" F., and then introduced into phase separator 78.The refrigerant vapor and refrigerant liquid in said phase separator 78are handled preferred embodiment, a refrigerant liquid is employed' atits highest refrigeratingtemperature in segmentor region (b) of heatexchanger 12 where, because of its higher pressure and highest propanecontent (about 91 percent), it effects the'f rst cooling of the warmincoming streams in said conduits 10 and 52. To obtain a lowertemperature refrigerant for additional cooling of I said incomingstreams, a second stream of the refrigerant liquid employed in region(b) of said heat exchanger 12 is expanded or flashed through expansionvalve 76 to thereby obtain a lower boiling mixture, which by virtue ofits reduced pressure is then employedin said second region or section(c) of said heat exchanger 12. In order to obtain a still lowertemperature refrigerant for use in cooling said incoming streams totheir lowest desired temperature, the gaseous refrigerant from-storagetankor surge tank 50 is employed which, because of its increased ethanecontent (about 3| percent) boils at the lowest temperature afterliquefaction by'passa'ge through said heat exchanger l2 and flashing orexpanding same through said expansion valve 96.

Said presently preferred embodiment of the invention thus provides threetemperature levels of refrigeration by employing two-stage flashing of arefrigerant liquid to obtain the high temperature and ntermediatetemperature levels of cooling, and employing a singlestage condensationof refrigerant vapor, which refrigerant vapor after liquefaction isexpanded to obtain the lowest stage of cooling. I

Referring now to FIG. 2, there is illustrated another embodiment of theinvention which can be employed for recovering a mixture of ethane andpropane from a mixture of methane, ethane, and heavier hydrocarbonsextracted from a natural gas stream by cooling and partially liquefyingsaid natural gas stream. Said mixture of ethane and propane canconveniently be employed as the multicomponent refrigerant employed inthe refrigeration system illustrated in FIG. 1.

A mixture of partially liquefied hydrocarbons comprising methane,ehtane, and heavier hydrocarbons,

having been recovered from a natural gas stream as described inconnection with FIG. 1, is passed via conduit 122 into a firstfractionation zone or deethanizer 124 as feedstock thereto. In saiddeethanizer, said feedstock is fractionated to recover an overheadstream comprising methane, ethane, and some propane. Typical operatingconditions in fractionator 124 would include a top tower temperature ofabout 1 1 F., a bottom tower temperature of about 219 F., and a pressureof 412 psig. Said overhead stream is removed via conduit 126, partiallycondensed in condenser 128, and passed into accumulator 130. A vaporstream comprising methane is removed from said accumulator 130 viaconduit 132. A first portion of the condensed overhead is removed fromaccumulator 130 and passed via conduit 134 into said fractionator 124near the top thereof, as reflux liquid. Preferably, the amount of saidreflux passed via conduit 134 is set at a predetermined amount andcontrolled by rate of flow controller 136. A second portion of saidcondensed overhead is removed from said accumulator 130 and passed viaconduit 138 to a second fractionator or demethanizer 140 as feedstockthereto. Typical operating conditions in fractionator 140 would includea top tower temperature of about 1 6 F., a bottom tower temperature ofabout 46 F. and a'pressure of about 412 psig. An overhead stream iswithdrawn from fractionator 140 via conduit 142, passed into conduit 126wherein its combined with the overhead from said first fractionator 124,and the total stream passed through said cooler 128 wherein it is cooledto a temperature of about F. and thereby partially condensedorliquefied. Any

If the embodiment of the invention illustrated in FIG.

'2 is being employed in combination with the embodiment of the inventionillustrated in FIG. 1, said refrigerant can conveniently be a portion ofthe refrigerant in conduit in said FIG. 1. Said accumulator thus servesas a common accumulator for both said fractionator 124 and saidfractionator 140. Thus, the same liquid is used as reflux infractionator 124 and as feedstock in fractionator 140. Preferably, theamount of said feedstock passed to fractionator is controlled inaccordance with the liquid level in reboiler 144. Flow of said feedstockcan be regulated by means of valve 146 responsive to a signal fromliquid level controller 148. A product mixture comprising ethane andpropane is withdrawn from reboiler region 144 of fractionator 140 bymeans of conduit 150. If desired, the rate of withdrawal of said productstream can be controlled by means of a rate of flow controller, notshown. A second product of the process is withdrawn as bottoms productfrom fractionator 124, being withdrawn from reboiler region 152 viaconduit 154. The rate of withdrawal of said second product can becontrolled by valve 156 responsive to a signal from liquid levelcontroller 158.

Said fractionator 124 can be conveniently reboiled by means of a hotheat exchange medium supplied by pipe or conduit 160 to reboiler region152. The flow rate of said hot heat exchange medium can be controlled byvalve 162 which can be adjusted or regulated by temperature recordercontroller 164 responsive to a temperature measurement in the lowerportion of 6 fractionator 124, which is transmitted by transmitter 166.The set point of temperature recorder controller 164 can be in turnregulated responsive to a signal from liquid level controller 168 onphase separator 130. Thus, said fractionator 124 will be reboiledsufficiently to maintain a desired liquid level in phase separator 130,i.e., to maintain sufficient liquid to supply reflux to saidfractionator 124 and feedstock to said fractionator 140.

Said fractionator 140 can be advantageously reboiled by passing aportion of the warm bottoms stream (about 218 F.) from reboiler region152 via conduits 154 and 170 with the flow rate thereof being controlledby valve 172 which is regulated by temperature recorder controller 174responsive to the measured temperature (about 46 F.) in reboiler region144. Effluent heating fluid from reboiler region 144 is returned toconduit 154 via conduit 176 and removed from the system with the productin said conduit 154. The ethanepropane refrigerant mixture recovered asproduct via conduit 150 will typically comprise about 60 percent ethaneand about 40 percent propane. However, it is within the scope of theinvention to recover mixtures having different proportions of saidcomponents. For example, the composition of said refrigerant mixture canvary from about 25 mol percent to about 95 mol percent ethane and fromabout 75 percent to about 5 percent propane.

The embodiment of the invention illustrated in FIG. 2 thus provides aprocess for recovering a desired mixture of ethane and propane from theoverhead of a deethanizer in a cryogenic natural gas processing plant.The recovery of said refrigerant mixture is effected in a demethanizingfractionator which is refluxed and fed with part of the condensedoverhead stream from said deethanizer. Further, the demethanizingfractionator producing the desired refrigerant mixture is reboiled withbottoms from said deethanizer, thereby saving reboiling costs. Thus, thefractionation method by which said refrigerant mixture is producedrequires neither separate reflux facilities nor external heat forreboiling.

While the invention has been describedwith particular reference tothecooling of a natural gas in a heat exchange zone, the invention isnot limited thereto. The

refrigeration system of the invention can be employed to cool-othermaterials contained in said zone. For example, the three regions of theheat exchange zone could comprise three refrigeration zones for storingdifferent materials at three different temperatures, e.g., threedifferent refrigeration rooms in a large cold storage plant.

While'certain embodiments of the invention have been described forillustrative purposes, the invention is notlimited thereto. Variousother modifications or embodiments of the invention will be apparent tothose skilled in the art in view of this disclosure. Such modificationsor embodiments are within the spirit and scope of the disclosure.

We claim:

1. A method for removing heat from a heat exchange zone and the contentsthereof, utilizing refrigerant vapor and refrigerant liquid streamsobtained from a compressed and partially liquefied refrigerant, whichmethod comprises:

passing a stream of said refrigerant vapor through a first region ofsaid heat exchange zone in indirect heat exchange relationship with atleast one expanded liquid stream so as to cool said refrigerant vapor,said expanded liquid stream derived from said refrigerant liquidexpanded to a first pressure and separated into expanded liquid andexpanded vapor producing said expanded liquid having a temperature lessthan the temperature of said resaid stream of refrigerant vapor so as tovaporize said refrigerant liquid and furthercool and liquefy saidrefrigerant vapor;

e. withdrawing a stream of said liquefied refrigerant vapor from saidheat exchange zone;

f. flashing said liquefied stream of step (d) to further cool same andform refrigerant vapor and refrigerfrigerant vapor; 5 ant liquid; and

passing said cooled refrigerant vapor through a sucg. introducing amixture of said refrigerant vapor and ceeding downstream region of saidheat exchang said refrigerant liquid obtained in said step (1') intoZone mdlreclheat f g relationship with said third region of said heatexchange zone as said anQtheY f of sf a Obtained a mixed stream ofrefrigerant vapor and refrigerant scribed hereinafter, said anotherstream of refrigerl i i f i step am at a temperatur'e less than p of 5.A method in accordance with claim 4 wherein: z tf f q r so as tofurthercoo] and q said refrigerant vapor of step (a) is withdrawn fromUs y 531 re l'lgel'am p a refrigerant storage zone;

flashmg 531d l'quefied refngeranivapof to a Pressure 1 5 refrigerantliquid is withdrawn from said storage zone a fd f f w whlch 3 3 one andflashed a first time to reduce the temperature 0 sm re ngeration iqur isexpan e so as to urand pressure thereof; ther cool same a to obtamalfother refrger' a first portion of said flashed refrigerant liquid isinant stream of m xed vapor and liquid; and. troduced into said heatexchange zone in step (b) rg f th heift 8 28 3 3; y i as said firstrefrigerant liquid; and

Z: g g gfx zi gg figg 2 :2 re 5 a going portion of! said flashsdLefngeraInt liquid is as e a secon time an t e resu ting twice 2. Amethod inaccordance with claim -1 wherein said fl e ashed refrigerantliquid isintroduced mto said regr gerant lfromdwhichbsa d rgfngeranltvapor and said heat exchange Zone in step (c) as Said secondgleigrgzrxant lqlll are 0 tame IS a mu tlcomponent refrigerant liqum 6.A method in accordance with claim 5 wherein:

3. A method n accordance wlth claim 2 wherein: vapors resulting fromSaid step (b) are withdrawn said vapors resulting from the vaporizationof said refrom said heat exchange zone, and compressed,

fngerams are gompressed cooled to paftlany vapors resulting from saidstep (c) are withdrawn z r ggg gfi gg 2335 3 ggd from said heat exchangezone and compressed;

. 9 vapors resulting from said step (d) are withdrawn mentionedrefrigerant liquid are withdrawn from Said storage Zone for use in Saidheat exchange from said heat exchange zone and compressed;and

zone. I i v 4. A method in accordance with claim 2 wherein said 3 5 ip ivapors ig i i q lmethod comprises, in combination, the steps of: y lqueSame an e resu mg we ls a of vapor 7 rzstzsifszzziazzsxrh 6 said heatexchange zone; v

b. introducing a first stream of said refrigerant liquid vapor 95 fromfirst g of i l at a temperature less than the temperature of said 40cram P' are P a stream of refrigerant vapor of step (a) into a first 9"?P o e resu tmg i m region of said heat exchange Zone and passing Sameuid IS withdrawn from said first separation zone and in' indirect heatexchange relationship with said utlhzed Ste p(b)and Vapors f f' mstream, of refrigerant vapol, of Step (a) so 3810 said first separat onzone are combined withsaid porize said refrigerant liquid and cool saidstream map Y flthdrawn heat if of refrigerant vapor; zone, prior to saidcompression; and

c. introducing a second stream of said refrigerantliqand fi fromsecond'flashmg of sad uid at a temperature less than the temperatureoffngeram hqu'd are P f to a PP? P said first stream of refrigerant liquidof step (11) into Zone resultmg I- quid W a second region of said heatexchange zone, downg m sale Second separatlonfom? and stream from saidfirst region with respect to the P "l said step and P 'Q flow of saidrefrigerant vapor, and passing same in i sccond sepamuofl Zone are f dWlth indirect heat exchange relationship with said i 5 P wmfdmwn saidheat stream of refrigerant vapor so as to vaporize said change P to saidP P refrigerant liquid and further cool said stream of A h d accordancewith clalm 6 wherein! f i t vapor; said step (d) vapors are compressedin a firststage of d. introducing a mixed stream of refrigerant vapor f-v g p Zone;

and refrigerant liquid, obtained as described here- 531d p i vapors andd PO m Said Second inafter, at a temperature less than the temperatureseparation o e e co th the compressed of said second stream ofrefrigerant liquid of step vapors from sald first'stage and theresulting mix- (6) into a third r i n f aid h at x ha one, turecompressed in a second stage of said compresdownstream from said secondregion with respect sion zone; to the flow of said refrigerant vapor,and passing said step' (b) vapors and said vapors from said first samein indirect heat exchange relationship with separation zone are combinedwith the compressed.

vapors from said second stage and the resulting mixture compressed in athird stage of said compression zone; and l the compressed vapors fromsaid third stage are cooled to partially liquefy same and the resultingmixture is passed to said storage zone.

9. A method in accordance with claim 4 wherein:

said refrigerant vapor of step (a) is withdrawn from a refrigerantstorage zone;

a first portion of refrigerant liquid withdrawn from said storage zoneis flashed to reduce the temperature and pressure thereof;

said flashed first portion of refrigerant liquid is introduced into saidheat exchange zone in step (b) as said first refrigerant liquid;

a second portion of refrigerant liquid withdrawn from said storage zoneis flashed a'first time to reduce the temperature and pressure thereof,and liquid refrigerant resulting from said first flashing is flashedasecondtimeto further reduce the temperature and pressure .thereof;and

said twice flashed second portion ofrrefrigerant liquid is introducedinto said heat exchange zone in step (c)..as said second refrigerantliquid.

'10. Amethod in accordance with claim-6 wherein:

said vapors resulting from step (c) are withdrawn from said .heatexchange zone after passage through said secondregion thereof; and

said vapors resulting from step (d) are withdrawn from said heatexchange .zone after passage through said third region thereof.

11. A method inaccordance with .claim 6 wherein:

said vapors resulting from step (c) are withdrawn from said heatexchange zone after passage through said second region and said'firstregion-of saidheat exchange zone.

12. A method inaccordance with .claim 6 wherein:

said vapors resulting from step (d) are withdrawn from said heatexchange vzone after passage throughsaid third region, said secondregion, and said first region of said heat exchangezone.

13. A method-in accordance withclaim6 wherein:

stream of step (i) as "another product of the process. 1

16. A method accordingto claim 15' wherein:

the temperature of said mixed stream of refrigerant vapor and liquidintroduced into said heat exchange zone in step (d) is adjusted bycontrolling the amount of said liquefied refrigerant vapor flashed instep (i) in accordance with the temperature of said partially liquefiedstream of natural gas withdrawn from said heat exchange zone in step (i)so as to maintain the final temperature on said partially liquefiedstream of natural gas at a predetermined value;

the amount of said liquefied refrigerant vapor flashed in step (I) isfurther controlled in accordance with the flow rate of said refrigerantvapor introduced into said heat exchange zone in step (a); and

.the ratio of the amount of vapor withdrawn from said first phaseseparation zone to the amount of said step (b) vapors withdrawn fromsaid heat exchange zone is maintained at a predetermined'value.

17. A method'according to claim 15 comprising, in

'further combination, the steps of:

l. passing said stream comprising methane, prior to recovery in step(i), through said heat exchange zone;

m. in step (k), flashing said'liquid stream of step (i) comprisingethane and higher boiling hydrocarbons to form a vapor and a liquid;

11. recombining said last-mentioned vapor and liquid, passing saidrecombined vapor and liquid through a portion of said heat exchange zoneand then introducing same as feedstock into a deethanizing zone; and r0.. recovering said streamcomprising propane and higher boilinghydrocarbons of step (k) from said deethanizing zone as another productof the process.

18. A method according to claim 17 wherein said liqsaid vapors.resulting from step (c) are withdrawn 40 uid stream of step (i) alsocontains some dissolved from said heat exchange zone after passagethrough said second region and said first region of heat exchange zone;and

said vapors resulting from step (d) are withdrawn from said heatexchange zone after passage through said third region, said secondregion, and said first region of said heat'exchange zone.

14. A method according to claim 4 comprising, in

further combination, the steps of:

h. passinga-stream ofgas through said heat exchange .zone in indirectheat exchange relationship with said streams of refrigerant vapor andrefrigerant liquids, and at least partially liquefying said stream ofgas.

15. A method according to claim 14 wherein said stream of gas in step(h) is a natural gas, and said :method comprises, in furthercombination,-the steps of:

i. withdrawing a partially liquefied stream of said natural gas fromsaid heat exchange zone and separating same into a vapor streamcomprising methane and a liquid stream comprising ethane and higherboiling hydrocarbons;

j. recovering said vapor stream-comprising methane of step (i) as oneproduct of the process; and

k. recovering a stream comprising propane and higher boilinghydrocarbons from said liquid methane, and said process comprises, infurther combination, the steps of:

p. in said deethanizing zone of step (n), fractionating said feedstockto recover an overhead stream comprising methane, ethane, and propane,and a bottoms stream comprising propane and higher boiling hydrocarbons;

:q. partially condensing said overhead stream from step (P); v q

r. returning a first portion of said condensed overhead of step (q) tosaid deethanizing zone as reflux thereto;

.s. passing a second portion of said condensed overhead of step (q) to ademethanizing zone as feedstock thereto; and A t. in said demethanizingzone, fractionating said feedstock to recover an overhead streamcomprising methane,.and a bottoms stream comprising ethane and propaneas another product of the process.

19. A method according to claim 18 wherein:

said overhead stream from said demethanizing zone is combined with saidoverhead stream from said deethanizing zone prior to said partialcondensation of step (q) and the resulting condensate is passed to acommon accumulator zone from which said portions of condensate of steps(r) and (s) are withdrawn;

a stream comprising methane is removed from said the amount of saidsecond portion of condensed common accumulator zone; overhead fed tosaid demethanizing zone is consaid bottoms stream from said deethanizerzone is retrolled in accordance with the liquid level in said moved froma reboiler region therein; reboiler region therein;

said bottoms product comprising ethane and propane the amount of warmliquid from said reboiler region 4 from said demethanizing zone isremoved from a of said deethanizer zone used to heat said reboilerreboiler region therein; and region of said demethanizing zone iscontrolled in said reboiler region of said demethanizing zone isaccordance with the temperature in said reboiler heated by indirect heatexchange with warm liquid region of said demethanizing zone; and

from said reboiler region of said deethanizing zone. the amount of heatsupplied to said reboiler region of said deethanizing zone is controlledin accordance 20. A method according to claim 19 wherein: with the levelin said common accumulator zone so the amount of said first portion ofcondensed overas to maintain a desired level in said common accuheadreturned to said deethanizing zone is set at a mulator zone.predetermined amount;

2. A method in accordance with claim 1 wherein said refrigerant fromwhich said refrigerant vapor and said refrigerant liquid are obtained isa multicomponent refrigerant.
 3. A method in accordance with claim 2wherein: said vapors resulting from the vaporization of saidrefrigerants are compressed, cooled to partially liquefy same, andpassed to a storage zone; and said first-mentioned refrigerant vapor andfirst-mentioned refrigerant liquid are withdrawn from said storage zonefor use in said heat exchange zone.
 4. A method in accordance with claim2 wherein said method comprises, in combination, the steps of: a.passing a stream of said refrigerant vapor through said heat exchangezone; b. introducing a first stream of said refrigerant liquid at atemperature less than the temperature of said stream of refrigerantvapor of step (a) into a first region of said heat exchange zone andpassing same in indirect heat exchange relationship with said stream ofrefrigerant vapor of step (a) so as to vaporize said refrigerant liquidand cool said stream of refrigerant vapor; c. introducing a secondstream of said refrigerant liquid at a temperature less than thetemperature of said first stream of refrigerant liquid of step (b) intoa second region of said heat exchange zone, downstream from said firstregion with respect to the flow of said refrigerant vapor, and passingsame in indirect heat exchange relationship with said stream ofrefrigerant vapor so as to vaporize said refrigerant liquid and furthercool said stream of refrigerant vapor; d. introducing a mixed stream ofrefrigerant vapor and refrigerant liquid, obtained as describedhereinafter, at a temperature less than the temperature of said secondstream of refrigerant liquid of step (c) into a third region of saidheat exchange zone, downstream from said second region with respect tothe flow of said refrigerant vapor, and passing same in indirect heatexchange relationship with said stream of refrigerant vapor so as tovaporize said refrigerant liquid and further cool and liquefy saidrefrigerant vapor; e. withdrawing a stream of said liquefied refrigerantvapor from said heat exchange zone; f. flashing said liquefied stream ofstep (d) to further cool same and form refrigerant vapor and refrigerantliquid; and g. introducing a mixture of said refrigerant vapor and saidrefrigerant liquid obtained in said step (f) into said third region ofsaid heat exchange zone as said mixed stream of refrigerant vapor andrefrigerant liquid of said step (d).
 5. A method in accordance withclaim 4 wherein: said refrigerant vapor of step (a) is withdrawn from arefrigerant storagE zone; refrigerant liquid is withdrawn from saidstorage zone and flashed a first time to reduce the temperature andpressure thereof; a first portion of said flashed refrigerant liquid isintroduced into said heat exchange zone in step (b) as said firstrefrigerant liquid; and a second portion of said flashed refrigerantliquid is flashed a second time and the resulting twice flashedrefrigerant liquid is introduced into said heat exchange zone in step(c) as said second refrigerant liquid.
 6. A method in accordance withclaim 5 wherein: vapors resulting from said step (b) are withdrawn fromsaid heat exchange zone and compressed; vapors resulting from said step(c) are withdrawn from said heat exchange zone and compressed; vaporsresulting from said step (d) are withdrawn from said heat exchange zoneand compressed; and said compressed vapors are combined, cooled topartially liquefy same, and the resulting mixture is passed to saidstorage zone.
 7. A method in accordance with claim 6 wherein: vapor andliquid from said first flashing of said refrigerant liquid are passed toa first phase separation zone, a first portion of the resultingseparated liquid is withdrawn from said first separation zone andutilized in said step (b), and vapors withdrawn from said firstseparation zone are combined with said step (b) vapors withdrawn fromsaid heat exchange zone, prior to said compression; and vapor and liquidfrom said second flashing of said refrigerant liquid are passed to asecond phase separation zone, the resulting separated liquid iswithdrawn from said second separation zone and utilized in said step(c), and vapors withdrawn from said second separation zone are combinedwith said step (c) vapors withdrawn from said heat exchange zone, priorto said compression.
 8. A method in accordance with claim 6 wherein:said step (d) vapors are compressed in a first stage of a three-stagecompression zone; said step (c) vapors and said vapors from said secondseparation zone are combined with the compressed vapors from said firststage and the resulting mixture compressed in a second stage of saidcompression zone; said step (b) vapors and said vapors from said firstseparation zone are combined with the compressed vapors from said secondstage and the resulting mixture compressed in a third stage of saidcompression zone; and the compressed vapors from said third stage arecooled to partially liquefy same and the resulting mixture is passed tosaid storage zone.
 9. A method in accordance with claim 4 wherein: saidrefrigerant vapor of step (a) is withdrawn from a refrigerant storagezone; a first portion of refrigerant liquid withdrawn from said storagezone is flashed to reduce the temperature and pressure thereof; saidflashed first portion of refrigerant liquid is introduced into said heatexchange zone in step (b) as said first refrigerant liquid; a secondportion of refrigerant liquid withdrawn from said storage zone isflashed a first time to reduce the temperature and pressure thereof, andliquid refrigerant resulting from said first flashing is flashed asecond time to further reduce the temperature and pressure thereof; andsaid twice flashed second portion of refrigerant liquid is introducedinto said heat exchange zone in step (c) as said second refrigerantliquid.
 10. A method in accordance with claim 6 wherein: said vaporsresulting from step (c) are withdrawn from said heat exchange zone afterpassage through said second region thereof; and said vapors resultingfrom step (d) are withdrawn from said heat exchange zone after passagethrough said third region thereof.
 11. A method in accordance with claim6 wherein: said vapors resulting from step (c) are withdrawn from saidheat exchange zone after passage through said second region and saidfirsT region of said heat exchange zone.
 12. A method in accordance withclaim 6 wherein: said vapors resulting from step (d) are withdrawn fromsaid heat exchange zone after passage through said third region, saidsecond region, and said first region of said heat exchange zone.
 13. Amethod in accordance with claim 6 wherein: said vapors resulting fromstep (c) are withdrawn from said heat exchange zone after passagethrough said second region and said first region of heat exchange zone;and said vapors resulting from step (d) are withdrawn from said heatexchange zone after passage through said third region, said secondregion, and said first region of said heat exchange zone.
 14. A methodaccording to claim 4 comprising, in further combination, the steps of:h. passing a stream of gas through said heat exchange zone in indirectheat exchange relationship with said streams of refrigerant vapor andrefrigerant liquids, and at least partially liquefying said stream ofgas.
 15. A method according to claim 14 wherein said stream of gas instep (h) is a natural gas, and said method comprises, in furthercombination, the steps of: i. withdrawing a partially liquefied streamof said natural gas from said heat exchange zone and separating sameinto a vapor stream comprising methane and a liquid stream comprisingethane and higher boiling hydrocarbons; j. recovering said vapor streamcomprising methane of step (i) as one product of the process; and k.recovering a stream comprising propane and higher boiling hydrocarbonsfrom said liquid stream of step (i) as another product of the process.16. A method according to claim 15 wherein: the temperature of saidmixed stream of refrigerant vapor and liquid introduced into said heatexchange zone in step (d) is adjusted by controlling the amount of saidliquefied refrigerant vapor flashed in step (f) in accordance with thetemperature of said partially liquefied stream of natural gas withdrawnfrom said heat exchange zone in step (i) so as to maintain the finaltemperature on said partially liquefied stream of natural gas at apredetermined value; the amount of said liquefied refrigerant vaporflashed in step (f) is further controlled in accordance with the flowrate of said refrigerant vapor introduced into said heat exchange zonein step (a); and the ratio of the amount of vapor withdrawn from saidfirst phase separation zone to the amount of said step (b) vaporswithdrawn from said heat exchange zone is maintained at a predeterminedvalue.
 17. A method according to claim 15 comprising, in furthercombination, the steps of: l. passing said stream comprising methane,prior to recovery in step (j), through said heat exchange zone; m. instep (k), flashing said liquid stream of step (i) comprising ethane andhigher boiling hydrocarbons to form a vapor and a liquid; n. recombiningsaid last-mentioned vapor and liquid, passing said recombined vapor andliquid through a portion of said heat exchange zone and then introducingsame as feedstock into a deethanizing zone; and o. recovering saidstream comprising propane and higher boiling hydrocarbons of step (k)from said deethanizing zone as another product of the process.
 18. Amethod according to claim 17 wherein said liquid stream of step (i) alsocontains some dissolved methane, and said process comprises, in furthercombination, the steps of: p. in said deethanizing zone of step (n),fractionating said feedstock to recover an overhead stream comprisingmethane, ethane, and propane, and a bottoms stream comprising propaneand higher boiling hydrocarbons; q. partially condensing said overheadstream from step (p); r. returning a first portion of said condensedoverhead of step (q) to said deethanizing zone aS reflux thereto; s.passing a second portion of said condensed overhead of step (q) to ademethanizing zone as feedstock thereto; and t. in said demethanizingzone, fractionating said feedstock to recover an overhead streamcomprising methane, and a bottoms stream comprising ethane and propaneas another product of the process.
 19. A method according to claim 18wherein: said overhead stream from said demethanizing zone is combinedwith said overhead stream from said deethanizing zone prior to saidpartial condensation of step (q) and the resulting condensate is passedto a common accumulator zone from which said portions of condensate ofsteps (r) and (s) are withdrawn; a stream comprising methane is removedfrom said common accumulator zone; said bottoms stream from saiddeethanizer zone is removed from a reboiler region therein; said bottomsproduct comprising ethane and propane from said demethanizing zone isremoved from a reboiler region therein; and said reboiler region of saiddemethanizing zone is heated by indirect heat exchange with warm liquidfrom said reboiler region of said deethanizing zone.
 20. A methodaccording to claim 19 wherein: the amount of said first portion ofcondensed overhead returned to said deethanizing zone is set at apredetermined amount; the amount of said second portion of condensedoverhead fed to said demethanizing zone is controlled in accordance withthe liquid level in said reboiler region therein; the amount of warmliquid from said reboiler region of said deethanizer zone used to heatsaid reboiler region of said demethanizing zone is controlled inaccordance with the temperature in said reboiler region of saiddemethanizing zone; and the amount of heat supplied to said reboilerregion of said deethanizing zone is controlled in accordance with thelevel in said common accumulator zone so as to maintain a desired levelin said common accumulator zone.